Wet strength is a desirable attribute of many disposable paper products, that must maintain their integrity for an extended period of time when wetted during their intended use. Such products include toilet and facial tissue, paper towels, and some of the components of diapers and adult incontinents, feminine hygiene products such as sanitary napkins, pantiliners and tampons, and the like.
A number of resins have been used or disclosed as being useful for providing wet strength to paper products and components thereof. Certain of these wet strength additives have resulted in paper products with permanent wet strength, i.e., paper which when placed in an aqueous medium retains a substantial portion of its initial wet strength over time. Exemplary resins of this type include urea-formaldehyde resins, melamine-formaldehyde resins and polyamide-epichlorohydrin resins. Such resins exhibit limited wet strength decay, even in the presence of excess water.
However, permanent wet strength in paper products is often an unnecessary and undesirable property. Indeed, due to the permanent wet strength of such products, or components thereof, paper products are generally disposed of after brief periods of use into landfill, incinerators, etc. Such products can therefore pose a significant burden on the solid waste stream. The desirable alternative of directing used paper products to municipal sewage treatment facilities or private septic systems is typically obviated by the inclusion of "unflushable" components, such as topsheets and backsheets. That is, clogging of these systems can result if the product, or one or more of its components, permanently retains hydrolysis-resistant strength properties. Therefore, efforts have been undertaken to provide paper product components, and specifically incontinent and personal hygiene product components, that have sufficient wet integrity when wetted with aqueous body fluids during use, but which lose their integrity when exposed to large amounts of waste water (such as is encountered in a typical toilet) such that they traverse plumbing and disintegrate in municipal/septic systems. Numerous approaches for providing paper products claimed as having good initial wet strength which decays significantly over time have been suggested. For example, various approaches suggested to achieve temporary wet strength are described in U.S. Pat. No. 3,556,932, Coscia et al., issued Jan. 19, 1971; U.S. Pat. No. 3,740,391, Williams et al., issued Jun. 19, 1973; U.S. Pat. No. 4,258,849, Miller, issued Mar. 31, 1981; U.S. Pat. No. 3,096,228, Day et al., issued Jul. 2, 1983; U.S. Pat. No. 4,605,702, Guerro et al., issued Aug. 12, 1986; U.S. Pat. No. 4,675,394, Solarek, et al., issued Jun. 23, 1987; U.S. Pat. No. 5,509,913, issued Apr. 23, 1996 to Yeo; U.S. Pat. No. 4,603,176, Bjorkquist et al., issued Jul. 29. 1986; U.S. Pat. No. 4,981,557, Bjorkquist, issued Jan. 1, 1991; and U.S. Pat. No. 5,138,002, Bjorkquist, issued Aug. 11, 1992.
While the art has provided a variety of synthetic nonwoven products having varying degrees of wet strength, none has provided paper products that exhibit the combined in-use wet integrity and disposal decay properties of the present structures. In particular, existing temporary wet strength products exhibit immediate and rapid tensile decay upon exposure to aqueous fluids. Obviously, rapid decay in the presence of body fluids renders these materials unsuitable for use in absorbent articles which must retain their strength up until the time of disposal. Conversely, as discussed above, those paper products possessing more permanent wet strength will not decay even when exposed to large quantities of aqueous liquids.
It is therefore an object of this invention to provide flushable fibrous structures in the form of either paper-based or synthetic nonwovens that provide initial tensile strength even when subjected to aqueous body fluids, but which decay rapidly in the presence of excess amounts of water encountered during disposal. These structures may be in the form of a paper product such as toilet tissue, facial tissue, tissue towels and the like, or they may be used as a component in personal products, such as absorbent article topsheets, backsheets and the like.